System and method for preparing a container loaded with wet radioactive elements for dry storage

ABSTRACT

A system for preparing a container holding radioactive waste for dry storage. In one aspect, the invention can be a system for preparing a container having a cavity loaded with radioactive elements for dry storage, the system comprising: a gas circulation system comprising a condenser module, a desiccant module, and a gas circulator module; the gas circulation system configured to form a hermetically sealed closed-loop path when operably connected to the cavity of the container; and means for adding and removing the desiccant module as part of the hermetically sealed closed-loop path.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/886,844 filed Oct. 19, 2015, which in turn is a continuationof U.S. patent application Ser. No. 14/060,384, filed Oct. 22, 2013, nowU.S. Pat. No. 9,165,690, which in turn is a divisional of U.S.Non-provisional patent application Ser. No. 12/342,022, filed Dec. 22,2008, now U.S. Pat. No. 8,561,318, which in turn claims the benefit ofU.S. Provisional Patent Application Ser. No. 61/016,151, filed Dec. 21,2007, the entireties of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods ofpreparing a container loaded with wet radioactive elements, such as amulti-purpose canister or a thermally conductive cask, for dry storage,and specifically to a closed-loop system and method of drying amulti-purpose canister for dry storage using a forced gas flow.

BACKGROUND OF THE INVENTION

In the operation of nuclear reactors, hollow zircaloy tubes filled withenriched uranium, known as fuel assemblies, are burned up inside thenuclear reactor core. It is customary to remove these fuel assembliesfrom the reactor after their energy has been depleted down to apredetermined level. Upon depletion and subsequent removal, this spentnuclear fuel (“SNF”) is still highly radioactive and producesconsiderable heat, requiring that great care be taken in its subsequentpackaging, transporting, and storing. Specifically, the SNF emitsextremely dangerous neutrons and gamma photons. It is imperative thatthese neutrons and gamma photons be contained at all times subsequent toremoval from the reactor core.

In defueling a nuclear reactor, it is common place to remove the SNFfrom the reactor and place the SNF under water, in what is generallyknown as a spent fuel pool or pond storage. The pool water facilitatescooling the SNF and provides adequate radiation shielding. The SNF isstored in the pool for a period long enough to allow the decay of heatand radiation to a sufficiently low level to allow the SNF to betransported with safety. However, because of safety, space, and economicconcerns, use of the pool alone is not satisfactory where the SNF needsto be stored for any considerable length of time. Thus, when long-termstorage of SNF is required, it is standard practice in the nuclearindustry to store the SNF in a dry state subsequent to a brief storageperiod in the spent fuel pool, i.e., storing the SNF in a dry inert gasatmosphere encased within a structure that provides adequate radiationshielding. One typical structure that is used to store SNF for longperiods of time in the dry state is a storage cask.

Storage casks have a cavity adapted to receive a canister of SNF and aredesigned to be large, heavy structures made of steel, lead, concrete andan environmentally suitable hydrogenous material. However, because thefocus in designing a storage cask is to provide adequate radiationshielding for the long-term storage of SNF, size and weight are oftensecondary considerations (if considered at all). As a result, the weightand size of storage casks often cause problems associated with liftingand handling. Typically, storage casks weigh more than 100 tons and havea height greater than 15 ft. A common problem associated with storagecasks is that they are too heavy to be lifted by most nuclear powerplant cranes. Another common problems is that storage casks aregenerally too large to be placed in spent fuel pools. Thus, in order tostore SNF in a storage cask subsequent to being cooled in the pool, theSNF is transferred to a cask, removed from the pool, placed in a stagingarea, dewatered, dried, and transported to a storage facility. Adequateradiation shielding is needed throughout all stages of this transferprocedure.

As a result of the SNF's need for removal from the spent fuel pool andadditional transportation to a storage cask, an open canister istypically submerged in the spent fuel pool. The SNF rods are then placeddirectly into the open canister while submerged in the water. However,even after sealing, the canister alone does not provide adequatecontainment of the SNF's radiation. A loaded canister cannot be removedor transported from the spent fuel pool without additional radiationshielding. Thus, apparatus that provide additional radiation shieldingduring the transport of the SNF is necessary. This additional radiationshielding is achieved by placing the SNF-loaded canisters in largecylindrical containers called transfer casks while still within thepool. Similar to storage casks, transfer casks have a cavity adapted toreceive the canister of SNF and are designed to shield the environmentfrom the radiation emitted by the SNF within.

In facilities utilizing transfer casks to transport loaded canisters, anempty canister is first placed into the cavity of an open transfer cask.The canister and transfer cask are then submerged in the spent fuelpool. Prior to cask storage, the SNF is removed from the reactor andplaced in wet storage racks arrayed on the bottom of spent fuel pools.For dry storage, the SNF is transferred in the submerged canister thatis flooded with water and within the transfer cask. The loaded canisteris then fitted with its lid, enclosing the SNF and the water from thepool within. The loaded canister and transfer cask are then removed fromthe pool by a crane and set down in a staging area to prepare theSNF-loaded canister for long-term dry storage. In order for anSNF-loaded canister to be properly prepared for dry storage, the UnitedStates Nuclear Regulatory Commission (“N.R.C.”) requires that the SNFand interior of the canister be adequately dried before the canister issealed and transferred to the storage cask. Specifically, N.R.C.regulations mandate that the vapor pressure (“vP”) within the canisterbe below 3 Torrs (1 Torr=1 mm Hg) before the canister is backfilled withan inert and sealed. Vapor pressure is the pressure of the vapor over aliquid at equilibrium, wherein equilibrium is defined as that conditionwhere an equal number of molecules are transforming from the liquidphase to gas phase as there are molecules transforming from the gasphase to liquid phase. Requiring a low vP of 3 Torrs or less assuresthat an adequately low amount of moisture exists in the interior of thecanister and on the SNF so that the SNF is sufficiently dry forlong-term storage.

Currently, nuclear facilities comply with the N.R.C.'s 3 Torr or less vPrequirement by performing a vacuum drying process. In performing thisprocess, the bulk water that is within the canister is first drainedfrom the canister. Once the bulk of the liquid water is drained, avacuum system is coupled to the canister and activated so as to create asub-atmospheric pressure condition within the canister. Thesub-atmospheric condition within the canister facilities evaporation ofthe remaining liquid water while the vacuum helps remove the watervapor. The vP within the canister is then measured by placingappropriate measuring instruments, such as vacuum gages, into thecanister and taking direct measurements of the gaseous contents presenttherein. If necessary, this vacuum procedure is repeated until a vP of 3Torrs or less is obtained. Once an acceptable vP is reached, thecanister is backfilled with an inert gas and the canister is sealed. Thetransfer cask (with the canister therein) is then transported to aposition above a storage cask and the SNF-loaded canister is loweredinto the low storage for long-term storage.

Current methods of satisfying the N.R.C.'s 3 Torrs or less vPrequirement are potentially dangerous, operationally time consuming,prone to error, subjects the SNF rods to high temperatures, and costly.First, the intrusive nature of the direct vP measurement is dangerousbecause the canister contains highly radioactive SNF. Any time thecanister must be physically breached, there is the danger of exposingthe surrounding an environment and the work personnel to radiation.Moreover, the prolonged creation of sub-atmospheric conditions in thecanister can cause complicated equipment problems. Finally, theoperational durations for vacuum drying are unacceptably long as vacuumdrying times on the order of days is quite common. The vacuum operationis prone to line freeze ups and ice formation inside canister which cangive false readings to the instruments. Lowering of the canisterpressure causes a progressive loss of the heat transfer medium (gasfilling the gaps and open spaces in the canisters) resulting insubstantial elevation of temperature of heat producing SNF rods.

One of the major disadvantages of existing vacuum drying systems andmethods is that the SNF cladding heats up to unacceptable temperaturesthat may compromise the fuel cladding integrity. In order for liquidwater to be removed from the SNF canister using the existing vacuumdrying process, the canister must be held at a low vacuum level for anextended period while the liquid water boils off. The extended period oftime when the fuel is surrounded by a near vacuum impedes removal of thedecay heat from the fuel itself.

Recently, the assignee of the present application, Holtec International,Inc., has developed new and improved methods, apparatus and systems forpreparing canisters of spent nuclear fuel for dry storage utilizingforced gas dehydration (“FGD”). These inventions are fully described anddisclosed in U.S. Pat. No. 7,210,247, issued May 1, 2007, Krishna Singhand United States Patent Application Publication 2006/0272175A1,published Dec. 7, 2006, Krishna Singh, the entireties of which areincorporated herein by reference.

It has been discovered that the FGD drying methods, apparatus andsystems disclosed in U.S. Pat. No. 7,210,247 and United States PatentApplication Publication 2006/0272175A1 can be improved and/or simplifiedin a novel and non-obvious manner Referring to FIG. 3, the FGDtechnologies disclosed in the aforementioned references consist of anair or liquid cooled condenser module, a freeze drying module, acirculator module, and a pre-heater module to continuously circulate aninert gas through a spent nuclear fuel (“SNF”) canister in order toremove liquid moisture and dehumidify the gas that is ultimately sealedwithin the canister for transportation and storage. These systemsoperate to first remove the liquid moisture in the canister and then todehumidify the circulating gas stream prior to sealing the SNF canister.The FGD system uses a low temperature refrigerant system and heatexchanger to cool the circulating gas stream to the point where thewater vapor in it freezes onto the heat exchanger surface. The freezingof the water vapor on the exchanger surface acts to dehumidify thecirculating gas stream. It is proposed that the following modificationcan be used as alternatives to the freeze dryer module.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for drying a canister loaded with a high level radioactivewaste (“HLW”), such as SNF.

Another object of the present invention is to provide a method andsystem for drying a canister loaded with HLW without physicallyaccessing the contents of the canister to ensure that an acceptablylevel of dryness has reached within the canister.

Yet another object of the present invention is to provide a method andsystem for drying a canister loaded with HLW without subjecting theinterior of the canister to prolonged sub-atmospheric conditions.

A further object of the present invention is to provide a method andsystem for preparing an SNF-loaded canister for dry storage that is easyto implement and/or time efficient.

A yet further object of the present invention is to provide a method andsystem for preparing a canister loaded with HLW for dry storage in amore cost effective and safer manner.

In one aspect, the invention is a method of preparing a canister havinga cavity loaded with wet radioactive elements for dry storage, themethod comprising: (a) providing a gas circulation system comprising acondensing module, a desiccant module, a gas circulator module; (b)connecting the gas circulation system to the canister so as to form ahermetically sealed closed-loop path that includes the cavity; (c)filling the hermetically sealed closed-loop path with a non-reactivegas; (d) circulating the non-reactive gas through the hermeticallysealed closed-loop path until the condensing module is no longerremoving substantial amounts of water from the circulating non-reactivegas, wherein the desiccant module is sealed off from the hermeticallysealed closed-loop path during step (d); and (e) adding the desiccantmodule to the hermetically sealed closed-loop path and continuing tocirculate the non-reactive gas through the hermetically sealedclosed-loop path, the desiccant module dehumidifying the circulating thenon-reactive gas.

In another aspect, the invention can be a system for preparing acanister having a cavity loaded with radioactive elements for drystorage, the apparatus comprising: a gas circulation system comprising asource of a condenser module, a desiccant module, a gas circulatormodule; the gas circulation system adapted to form a hermetically sealedclosed-loop path when operably connected to the cavity of the canisterto be prepared for dry storage; and means for adding and removing thedesiccant module as part of the hermetically sealed closed-loop path.

In yet another aspect, the invention can be a method of preparing acanister having a cavity loaded with wet radioactive elements for drystorage, the method comprising: (a) providing a gas circulation systemcomprising a condensing module, a vacuum module, a gas circulatormodule; (b) connecting the gas circulation system to the canister so asto form a hermetically sealed closed-loop path that includes the cavity;(c) filling the hermetically sealed closed-loop path with a non-reactivegas; (d) circulating the non-reactive gas through the hermeticallysealed closed-loop path until the condensing module is no longerremoving substantial amounts of water from the circulating non-reactivegas, wherein the vacuum module is sealed off from the hermeticallysealed closed-loop path during step (d); (e) discontinuing thecirculation of the non-reactive gas through the hermetically sealedclosed-loop path; (f) fluidly coupling the vacuum module to the cavityand fluidly isolating the cavity and the vacuum module; and (g) applyinga vacuum pressure to the cavity via the vacuum module so as to create asub-atmospheric pressure within the cavity until a desired vaporpressure is achieved in the cavity of the canister

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art FGD system.

FIG. 2 is a perspective view of an embodiment of a prior artmulti-purpose canister (“MPC”) that can be used in conjunction with thepresent invention shown partially in section and empty.

FIG. 3 is a perspective view of a prior art transfer cask partially insection with the MPC of FIG. 2 sealed and positioned in the transfercask.

FIG. 4 is a schematic diagram of an FGD system according to oneembodiment of the present invention that utilizes a desiccant module todehumidify the circulating gas stream.

FIG. 5 is a schematic diagram of an FGD system according to anotherembodiment of the present invention that utilizes a vacuum module todehumidify the MPC.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is an improvement over the drying methods,apparatus and systems disclosed in U.S. Pat. No. 7,210,247 and UnitedStates Patent Application Publication 2006/0272175A1. The followingenhancements are proposed for the FGD drying systems for use in dryingcontainers designed for dry storage of high level radioactive waste,such MPSc loaded with SNF.

FIG. 1 illustrates a canister 20 that is suitable for use with thepresent invention. The present invention is not limited to specificcanister geometries, structures, or dimensions and is applicable to anytype of enclosure vessel used to transport, store, or hold radioactiveelements. While the exemplified embodiment of the invention will bedescribed in terms of its use to dry a canister of spent nuclear fuel(“SNF”), it will be appreciated by those skilled in the art that thesystems and methods described herein can be used to dry radioactivewaste in other forms and in a variety of different containmentstructures as desired.

The canister 20 comprises a bottom plate 22 and a cylindrical wall 24which forms a cavity 21. As used herein, the end 25 of the canister 20that is closest to the bottom plate 22 will be referred to as the bottomof the canister 20 while the end 26 of the canister 20 that is furthestfrom the bottom plate 22 will be referred to as the top of the canister20. The cavity 21 has a honeycomb grid 23 positioned therein. Thehoneycomb grid 23 comprises a plurality of rectangular boxes adapted toreceive spent nuclear fuel (“SNF”) rods. The invention is not limited bythe presence of the honeycomb grid.

The canister 20 further comprises a drain pipe with an open bottom (notillustrated) located at or near the bottom of the canister 20 thatprovides a sealable passageway from outside of the canister 20 to theinterior of the cavity 21. If desired, the drain opening can be locatedin the bottom plate 22 or near the bottom of the canister wall. Thedrain pipe can be opened or hermetically sealed using conventionalplugs, drain valves, or welding procedures.

As illustrated in FIG. 2, the canister 20 is empty (i.e. the cavity 21does not have SNF rods placed in the honeycomb grid 23) and the top 26of the canister 20 is open. In utilizing the canister 20 to transportand store SNF rods, the canister 20 is placed inside a transfer cask 10(FIG. 2) while the canister 20 is open and empty. The open transfer cask10, which is holding the open canister 20, is then submerged into aspent fuel pool which causes the volume of the cavity 21 to becomefilled with water. SNF rods that are removed from the nuclear reactorare then moved under water from the spent fuel pool and placed insidethe cavity 21 of the canister 20.

Preferably, a single bundle of SNF rods is placed in each rectangularbox of the honeycomb grid 23. Once the cavity 21 is fully loaded withthe SNF rods, the canister lid 27 (FIG. 3) is positioned atop thecanister 20. The canister lid 27 has a plurality of sealable lid holes28 that form a passageway into the cavity 21 from outside of thecanister 20 when open. The transfer cask 10 (having the loaded canister20 therein) is then lifted from the spent fuel pool by a crane andplaced uprightly in a staging area (as shown in FIG. 3) so that thecanister 20 can be properly prepared for dry-storage. This dry-storagepreparation includes drying the interior of the canister 20 and sealingthe lid 27 thereto.

Referring now to FIG. 3 exclusively, when in the staging area, thecanister 20 (containing the SNF rods and pool water) is within thetransfer cask 10. Both the canister 20 and the transfer cask 10 are inan upright position. Once in the staging area, the drain pipe attachedto the canister lid 27 (not illustrated) with a bottom opening at ornear the bottom 25 of the canister 20 is used to expel the bulk waterthat is trapped in the cavity 21 of the canister 20 using a blowdown gas(usually helium or nitrogen). Despite draining the bulk water from thecavity 21, residual moisture remains in the cavity 21 and on the SNFrods. However, before the canister 20 can be permanently sealed andtransported to a storage cask for long-term dry storage ortransportation, it must be assured that that cavity 21 and the SNF rodscontained therein are adequately dried.

Because a low vapor pressure (“vP”) within a container indicates that alow level of moisture is present, the United States Nuclear RegulatoryCommission (“NRC”) requires compliance to the 3 Torr or less vaporpressure (“vP”) specification within the cavity 21 of HLW containingcasks.

FIG. 4 is a schematic of an embodiment of an FGD system 300 capable ofdrying the cavity 21 to acceptable NRC levels without the need tointrusively measure the resulting vP within the cavity 21. Once thetransfer cask 10, which is holding the canister 20, is positioned in thestaging area and the bulk water is drained form the cavity 21, thedrying system 300 is connected to the inlet 28 and outlet 29 of thecanister 20 so as to form a closed-loop system. The closed-loop may ormay not include the desiccant module 370 depending on the status of thethree-way valves 421,422. The gas supply line 325 is fluidly connectedto the inlet 28 of the canister 20 while the gas exhaust line 326 isfluidly connected to the outlet 29 of the canister 20. The inlet 28 andoutlet 29 of the canister are mere holes in the canister 20. If desired,proper port connections, seals, and/or valves can be incorporated intothe inlet and outlet 28, 29.

The drying system 300 generally comprises a non-reactive gas reservoir310, a gas circulator module 320, a plurality of two-way valves 321-323,a plurality of three-way valves 421-422, a dew point temperaturehygrometer 330, a condensing module 340, a pre-heater module 380, adesiccant module 370, and a control system 350, which includes asuitably programmed microprocessor 351, a computer memory medium 352, atimer 353, and an alarm 354. While the illustrated embodiment of thedrying system 300 is automated via the control system 350, neither themethod nor system of the present invention is so limited. If desired,the functions carried out by the control system 350 can be carried outmanually and/or omitted in some instances.

The helium reservoir 320, the pre-heater module 380, the gas circulatormodule 320, the canister 20, the hygrometer 330, condensing module 340,and the desiccant module 370 are fluidly connected so that anon-reactive gas, such as helium, can flow through the drying system 300without escaping into the external environment. All of the gas linesconnecting the aforementioned component can be formed of suitable tubingor piping. The piping and tubing can be constructed of flexible ornon-flexible conduits. The conduits can be formed of any suitablematerial, such as metals, alloys, plastics, rubber, etc. All hermeticconnections can be formed through the use of threaded connections,seals, ring clamps, and/or gaskets.

The helium gas reservoir 310 is used to store pressurized helium gas andfeed helium gas to the loop for circulation by opening the valve 323.While helium gas is the preferred non-reactive gas for use in thepresent invention, any non-reactive gas can be used in conjunction withthe system 300 and the operation thereof. For example, other suitablenon-reactive gases include, without limitation, nitrogen,carbon-dioxide, light hydrocarbon gases such as methane, or any inertgas, including but not limited to noble gases (helium, argon, neon,radon, krypton and xenon).

When valve 323 is opened, the helium reservoir fills the closed loopwith helium. The gas circulator 320 is operably coupled to the gassupply line 325. The position of the gas circulator 320 in the loop canbe varied as desired. When activated, the gas circulator 320, which canbe a blower, forces helium gas through the closed-loop (which includesthe canister 20) at the desired flow rate. While a single gas circulator320 is illustrated as being incorporated into the drying system 300, theinvention is not so limited and any number of circulator or pumps can beused. The exact number of pumps will be dictated on a case-by casedesign basis, considering such factors as flow rate requirements,pressure drops in the system, size of the system, and/or number ofcomponents in the system. The direction of the helium gas flow throughsystem 300 is indicated by the arrows on the fluid lines.

Valve 321, 322 are operably coupled to the gas supply line 325 and thegas exit line 326 respectively. The valves 321, 322 are used to controlthe flow to the cavity 21 of the canister 20. Specifically, the valves321, 322 can be used to isolate the canister 20 from the rest of theloop when desired, such as during connection and disconnection. Allvalves used herein can be adjustable flow rate valves or simple on/offvalves. In other embodiments of the invention, mass flow ratecontrollers can be sued. As with the circulators, any number of valvescan be incorporated throughout the system 300 as desired. Only thosevalves considered important to the principles of the present inventionhave been illustrated. Moreover, the invention is not limited by anyspecific placement of the valve(s) or pump(s) along the closed-loop flowcircuit so long as the claimed methods can be performed.

The dew-point temperature hygrometer 330 is operably coupled to the gasexhaust line 326 so that the dew-point temperature of the helium gasexiting the cavity of the canister 20 can be measured. Suitable meansfor dew point temperature measurement include direct moisture sensingdevices, such as hygrometers, and other means, such as gaschromatography or mass spectroscopy. The hygrometer 330 preferablyincludes a digital signal in some embodiments. The dew point temperaturehygrometer 330 repetitively measures the dew point temperature of thehelium gas exiting the cavity 21. There is no requirement as to thesampling rate for repetitive measurements. For example, the dew pointtemperature hygrometer 330 can measure the dew point temperature of thehelium gas multiple times per second or only once every few minutes. Insome embodiments, the time intervals between repetitive measurementswill be so small that the measurements will appear to be essentiallycontinuous in nature (i.e., in real-time). The time intervals will bedetermined on case-by case design basis, considering such factors asfunctionality requirements of the system and the flow rate of the heliumgas.

The inlet 342 of the condenser module 340 is coupled to the gas exhaustline 326 while the outlet 343 is fluidly coupled to the recirculationline 345. The condenser module 340 is provided to adequatelyde-moisturize the wet helium gas that exits the cavity 21 of thecanister 20 during the liquid removal stage (Phase I) of drying thecanister 20. The helium gas leaving the condenser module 340 can bere-circulated back into the canister 20 after passing through thepreheater module 380 so that it can absorb more moisture. The condensermodule 340 is connected via drain 341 to a moisture accumulator 344(see, e.g. FIG. 4). The moisture accumulator can be monitored todetermine when Phase 1 is complete and the system 300 is ready for PhaseII drying (Phase II drying is the dehumidification of the circulatinggas stream prior to sealing the SNF canister). When monitoring themoisture accumulator 344, the end of Phase I is detected by no moremoisture/liquid accumulating in the reservoir of the moistureaccumulator, Phase I drying is complete. Alternatively, the hygrometer330 can be used to determine when Phase 1 is complete. When using thehygrometer 330, the end of Phase I is detected by the hygrometer 330obtaining a steady dew point measurement.

The desiccant module 370 is a pressure vessel or vessels containing asingle use or regenerative desiccant material. Candidate desiccantmaterials include Silica gel, Activated alumina, Molecular Sieve andsimilar hygroscopic type materials that would adsorb or absorb the watervapor from the gas stream. During Phase 1 drying, the desiccant module370 is valved out from the circulating gas stream by closing valves 421,422 so that the inlet line 371 and outlet line 372 of the desiccantmodule 370 is sealed from the recirculation line 345. This avoidsoverloading the desiccant materials with water. After the liquid waterhas been removed from the canister 20 and stripped from the circulatinggas by the condenser module 340 (i.e., Phase I drying is complete), thecirculating gas stream would be routed through the desiccant module 370by opening the valves 421, 422 so that the inlet and outlet lines 421,422 are in fluid communication with the recirculation line 345. Thedesiccant module 370 dehumidifies the circulating gas stream to theappropriate mass density prior to sealing the canister, therebycompleting Phase II drying.

The desiccant module 370 can be sized to dehumidify one or more SNFcanisters before the desiccant would need to be disposed of orregenerated. Water can be removed from the desiccant through aregenerative process, which consists of heating the desiccant materialto a known temperature and passing a dry gas such as air, nitrogen, orother inert gas over the desiccant bed. The desiccant can also be driedas necessary through heating, UV exposure, or other conventional dryingprocess and subsequently reused.

The drying system 300 further comprises an automation system 350. Thisis optional. The automation system 350 comprises a CPU 351, a computermemory medium 352, a timer 353, and an alarm 354. The CPU 351 is asuitable microprocessor based programmable logic controller, personalcomputer, or the like. The computer memory medium 352 can be a harddrive that comprises sufficient memory to store all of the necessarycomputer code, algorithms, and data necessary for the operation andfunctioning of the drying system 300, such as predetermined time,predetermined dew-point temperature, flow rates, and the like. The timer353 is a standard digitalized or internal computer timing mechanism. Thealarm 354 can be a siren, a light, an LED, a display module, a speaker,or other device capable of generating audio and/or visual stimulus.While an alarm 354 is illustrated and described, any instrumentation,device, or apparatus that inform an operator that the drying system 300has completed a drying process can be used. For example, a computerscreen can simply indicate that the canister is dry via text or visuals.

The CPU 351 includes various input/output ports used to provideconnections to the various components of the drying system 300 that needto be controlled and/or communicated with. The CPU 351 is operablycoupled to these components via electrical wires, fiber-optic lines,co-axial cables, or other data transmission lines. Wirelesscommunication can also be used. These connections are indicated by thedotted lines in FIG. 4. The CPU 351 can communicate with any and all ofthe various components of the drying system 300 to which it is operablyconnected in order to control the drying system 300, such as: (1)activating or deactivating the gas circulator 320; (2) opening, closing,and/or adjusting the valves 321-323, 421-422; (3) activating ordeactivating the condenser module 340 and the pre-heater 380; and (4)activating or deactivating the alarm 354.

The CPU 351 (and/or the memory 352) is also programmed with the properalgorithms to receive data signals from the dew-point hygrometer 330,analyze the incoming data signals, compare the values represented by theincoming data signals to stored values and ranges, and track the time atwhich the values represented by the incoming data signals are at orbelow the stored values. The type of CPU used depends on the exact needsof the system in which it is incorporated.

A method of preparing an MPC 20 loaded with wet SNF will now bedescribed according to an embodiment of the present invention isillustrated. The method will be described in relation to the dryingsystem 300 of FIG. 4 for ease of description and understanding. However,the method is not limited to any specific structure or system, and canbe carried out by other systems and/or apparatuses.

A cask 10 containing the SNF loaded canister 20 is positioned in astaging area after being removed from the cooling pool/pond. Asdiscussed above, the cavity 21 of the canister 20 is filled with waterfrom the pool at this time. The bulk water is drained from the cavity 21of the canister 20 via a properly positioned drain.

Despite the bulk water being drained from the cavity 21 of the canister20, the interior of the cavity 21 and the SNF are still moisture bearingand need further de-moisturization for long-term dry storage. In orderto further dry the cavity 21 and the SNF, the drying system 300 isutilized. The canister 20 remains in the cask 10 during the dryingoperation. The gas supply line 325 is fluidly coupled to the inlet 28 ofthe canister 20 while the gas exhaust line 326 is fluidly coupled to theoutlet 29 of the canister 20. As a result, a closed-loop fluid circuitis formed in which the cavity 21 of the canister 20 forms a portion ofthe fluid circuit when valves 321, 322 are opened. At this time, thevalves 421, 422 are in a position that seals the inlet and outlet lines371, 372 from the line 345, thereby removing the desiccant module 370from the main fluid circuit. Valve 323 is also closed at this time toavoid the wasted release of helium.

Once the drying system 300 is properly hooked up to the canister 20 theoperator activates the drying system 300. The drying system 300 can beactivated manually by switching on the equipment or in an automatedfashion by the CPU 351. When activated in an automated fashion, anoperator will activate the drying system 300 by entering a systemactivation command into a user input device (not illustrated), such as akeyboard, computer, switch, button, or the like, which is operablycoupled to the CPU 351. Upon receiving the associated system activationsignal from the user input device, the CPU 351 sends the appropriateactivation signals to the components of the system 300.

Valves 321, 322 are opened first. The valve 323 is then opened, therebyreleasing pressurized helium from the helium reservoir 30 that floodsthe closed-loop fluid circuit (which includes the gas supply line 325,the pre-heater module 380, the canister 20, the gas exhaust line 326,the condensing module 340, and the recirculation line 345). Thedesiccant module 370 is not part of the closed-loop fluid circuit atthis time. However, in an alternative embodiment, the desiccant module370 may be part of the closed-loop fluid circuit at this time to avoid apressure drop later when it is added to circuit after Phase I drying. Inthis scenario, the desiccant module 370 would be removed from thecircuit after it is filled with helium and before continuing with thegas circulation for Phase I drying.

Once the desired closed-loop circuit is filled with helium, valve 323 isclosed. The gas circulator 320 is then activated, along with thepre-heater module 280 and the condenser module 340, thereby circulatingthe helium gas through the fluid circuit. As a result, Phase I dryingbegins. The pre-heater 380 heats the helium before the entering thecanister 20 and the condenser module 340 removes moisture from thehelium that exits the canister 20.

The flow rate of the helium gas through the drying system 300 iscontrolled by either the gas circulator 320 or a flow rate valve. In oneembodiment to the present invention, the CPU 351 flows helium gasthrough the canister 20 at a flow rate of approximately 400 lb/hr.However, the invention is not so limited and other flow rates can beused. The exact flow rate to be used in any particular drying operationwill be determined on a case-by-case design basis, considering suchfactors as the open volume of the canister's cavity, the target drynesslevel within the canister's cavity, the initial moisture content withinthe canister's cavity, the moisture content of the helium gas maintainedwithin the reservoir, desired number of hourly volume turnovers for thecanister etc.

Upon being activated, the dry helium gas flows into the wet cavity 21 ofthe canister 20 via the inlet 28. Upon entering the cavity 21, the dryhelium gas absorbs water from the SNF and internal surfaces of thecavity 21 in the form of water vapor. The moisture laden helium gas thenexits the cavity 21 via the outlet 29. If the Phase I drying is beingmonitored by the hygrometer 330, the wet helium gas that exits thecavity 21 is repetitively measured by the hygrometer 330. As thehygrometer 330 measures the dew point temperature of the wetted heliumgas, it generates data signals indicative of the measured dew pointtemperature values and transmits these data signals to the CPU 351.Alternatively, if the Phase I drying is being monitored via anaccumulator coupled to the condenser, the hygrometer is not necessary atthis time and can be shut off.

As the wetted helium gas exits the canister 20 it enters the condensermodule 340, which has been activated by the CPU 351. The wetted heliumgas exiting the canister 20 is de-moisturized within the condenser 340prior to being re-circulated back to the pre-heater 380 via the line345. The liquid water condensed out of the helium gas within thecondenser module 340 drains out via the line 341 and into a moistureaccumulator where it is monitored to detect the end of Phase I drying.

The flow of helium through the circuit is continued until no more liquidis being condensed out by the condenser 340 (which is detected by eitherno more liquid accumulating in the moisture accumulator or a steadystate reading by the hygrometer 330), Phase I drying is determined to becomplete.

At this time, valves 421, 422 are open so that the inlet and outletlines 371, 372 are in fluid communication with the line 345, therebyadding the desiccant module to the loop/circuit. This begins Phase IIdrying, the dehumidification of the circulating helium gas stream priorto sealing the SNF canister. Once the desiccant module 370 has beenadded to the gas-circulation loop, the helium continues to be circulatedas in Phase I. However, the hygrometer 330 now becomes active (if notactive before) to determine the end of Phase II drying.

During Phase II, the hygrometer 330 is repetitively measuring the dewpoint of the wet helium gas that exits the cavity 21. As the hygrometer330 measures the dew point temperature of the wetted helium gas, itgenerates data signals indicative of the measured dew point temperaturevalues and transmits these data signals to the CPU 351. Upon receivingthe data signals indicative of the measured dew point temperaturevalues, the CPU 351 compares the measured values to a predetermined dewpoint temperature value that is stored in the memory medium 352. Thepredetermined dew point temperature is selected so as to be indicativethat the inside of the cavity 21 and the SNF is sufficiently dry forlong term storage. In one embodiment, the predetermined dew pointtemperature is selected so as to correspond to a vapor pressure in thecavity 21 that is indicative of an acceptable level of dryness, such asfor example 3 Torr or less. In such embodiments, the predetermined dewpoint temperature can be selected using either experimental or simulatedcorrelations.

An exemplary embodiment of how one selects the predetermined dew pointtemperature is described in United States Application Publication2006/0272175A1, published Dec. 7, 2006 to Krishna P. Singh. Theseteachings are incorporated by reference.

After the CPU 351 compares the measured dew point temperature to thepredetermined dew point temperature, the CPU 351 then determines whetherthe measured dew point temperature is less than or equal to thepredetermined dew point temperature. This comparison is performed foreach signal received by the CPU 351.

If the measured dew point temperature of the wetted helium gas exitingthe canister is determined to be above the predetermined dew pointtemperature, the CPU 351 will continue to determine whether the timer353 has been activated. If the timer 353 is activated, the CPU 351deactivates the timer 353 and returns to receiving data signals foranalysis. If the timer 353 is not activated, the CPU 351 returns toreceiving data. Either way, if the measured dew point temperature of thewetted helium gas exiting the canister is determined to be above thepredetermined dew point temperature, the drying system 300 continues tocirculate the dry helium gas into and through the cavity 21 of thecanister 20, thereby continuing Phase II drying.

However, if the measured dew point temperature of the wetted helium gasexiting the canister is determined to be at or below the predetermineddew point temperature, the CPU 351 will activate/start the timer 353.The timer 470 is programmed to run for a predetermined time. Theselection and purpose of the predetermined time will be discussed ingreater detail below.

Once the timer is activated, the CPU 351 proceeds to determine whetherthe timer 353 has expired (i.e., whether the predetermined time haspassed) without receiving a data signal indicative of a measured dewpoint temperature above the predetermined dew point temperature. If thisanswer is NO, the CPU 351 returns to the beginning and the drying system300 continues to circulate helium gas through the cavity 21 of thecanister 20 and repeat the operations of steps discussed above until thepredetermined time expires. In other words, the drying process continuesuntil the measured dew point temperature of the wetted helium gasexiting the canister falls below (or equal to) the predetermined dewpoint temperature, and remains so for the predetermined time (withoutsubsequently rising above the predetermined dew point temperature).

By requiring that the measured dew point temperature of the wettedhelium gas exiting the canister not only reach, but remain at or belowthe predetermined dew point temperature for the predetermined time, itis ensured that the cavity 21 and the SNF therein are sufficiently driedwithin an acceptable safety factor. This, along with the means forselecting the predetermined time, are described fully in United StatesApplication Publication 2006/0272175A1, published Dec. 7, 2006 toKrishna P. Singh. These teachings are incorporated by reference.

Once the predetermined time expires, and the measured dew pointtemperature remains at or below the predetermined dew point temperaturefor the entire predetermined time, the CPU 351 generates shut downsignals that are transmitted to the system 300. Upon receiving theshutdown signals, the circulator 320 is deactivated and the flow ofhelium gas through the drying system is ceased. The valves 321, 322 areclosed.

The CPU 351 generates and transmits an activation signal to the alarm354. Upon receiving the activation signal, the alarm 354 is activated.Depending on the type of device that is used as the alarm 354, theresponse of the alarm 354 to the activation signal can vary greatly.However, it is preferred that the alarm's 354 response be some type ofaudio and/or visual stimuli that will inform the operator that thecanister 20 is dry. For example, activation of the alarm 354 cangenerate a sound, display a visual representation on a computer screen,illuminate an LED or other light source, etc.

Upon being informed by the alarm 354 that the cavity 21 of the canister20 and the SNF is sufficiently dried, the operator disconnects thedrying system from the canister 20 and seals the canister 20 forstorage.

Referring now to FIG. 5, an FGD system, 500 according to secondembodiment of the present invention is disclosed. The FGD drying system500 is similar to the FGD drying system 100 discussed above in bothstructure and functioning. In order to avoid redundancy, only thoseaspects of FGD system 500 (and its functioning) that differ from the FGDsystem 100 will be discussed.

The FGD system 500 essentially replaces the desiccant module 370 of FGDsystem 100 with a vacuum module 400, which can be a conventional vacuumpump. The vacuum module 400 is downstream of valve 321 and upstream ofthe canister 20. The vacuum module 400 is operably coupled to the fluidcircuit and connected and disconnected through the valve 423.

When using the FGD system 500, the Phase I drying of the canister 200 isperformed in an essentially identical manner as described above for FGDsystem 100, wherein the vacuum module is isolated from thegas-circulation loop rather than the desiccant module.

While the FGD system 500 utilizes vacuum pressure to perform the PhaseII drying, it prevents the SNF cladding from heating up to unacceptabletemperatures that may compromise the fuel cladding integrity. In priorart vacuum systems, in order for liquid water to be removed from the SNFcanister, the canister must be held at a low vacuum level for anextended period while the liquid water boils off. The extended period oftime when the fuel is surrounded by a near vacuum impedes removal of thedecay heat from the fuel itself. However, in the FGD system 500 (and itsmethod) the time in which the canister 20 is subject to vacuum pressureis very short compared to conventional methods.

The FGD system 500 runs through the Phase 1 drying until all liquidwater is removed as discussed above. The circulating helium gas keepsthe SNF assemblies at a relatively low temperature during this process.The hygrometer 330 of the FGD system 500 is purely optional as it isonly used for the determination of the completion of Phase I drying. Itis not used in the Phase II operation.

Once Phase I is complete with the FGD system 500, the valves 321, 322are closed. The valve 423 is opened and the vacuum module 400 isactivated, thereby creating a sub-atmospheric condition within thecavity 21. The vacuum module 400 preferably evacuates the cavity 21, andholds the cavity 21 at less than 3 torr for 30 minutes to verify cavitydryness. Once the time is completed, the cavity is backfilled with aninert gas by proper manipulation of the valves. Because there is noresidual liquid water in the canister 20 after Phase I, the canistercavity 21 is rapidly evacuated (in 30 minutes or less) to a vaporpressure level below 3 torr without concerns about excessive water vaporflooding the vacuum system. Thus the time at low vacuum can be held to aperiod of less than 2 hours and therefore prevent unacceptably high fuelcladding temperatures.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in this art, the invention may be embodied in other specificforms without departing from the spirit or essential characteristicsthereof. Specifically, in some embodiments, the drying method of theinvention can be carried out manually. In such an embodiment, the pumpsand all other equipment will be activated/controlled manually. Thereadings by the hygrometer and the accumulator can be visually observedby the operator and the timing sequence operations can be performedmanually.

What is claimed is:
 1. A system for drying spent nuclear fuel in twophases, the system comprising: a canister defining a cavity configuredfor holding spent nuclear fuel elements; a gas circulation systemcomprising in fluid communication a condenser module and a gascirculator module; the gas circulation system fluidly connected to thecavity of the canister and configured to form a hermetically sealedclosed-loop flow path with the canister, the gas circulator moduleconfigured to circulate a non-reactive gas through the closed-loop flowpath for drying the spent nuclear fuel elements; an isolation valveoperable for selectively fluidly connecting a vacuum module to orisolating the vacuum module from the hermetically sealed closed-loopflow path; a programmable controller operably coupled to the isolationvalve, the gas circulator module, and the condensing module, theprogrammable controller configured to automatically: (1) isolate thevacuum module from the hermetically sealed closed-loop flow path duringa first drying phase; and (2) fluidly connect the vacuum module to thehermetically sealed closed-loop flow path during a second drying phase;wherein upon the programmable controller determining that the condensingmodule is no longer removing substantial amounts of water the firstdrying phase, the programmable controller: (1) fluidly isolates thecondensing module and gas circulator module from the hermetically sealedclosed-loop flow path; and (2) fluidly connects the vacuum module to thehermetically sealed closed-loop flow path to start the second dryingphase.
 2. The system according to claim 1, further comprising a moistureaccumulator which receives moisture from the condenser module, andwherein the programmable controller being operable to monitor themoisture accumulator and fluidly connect the vacuum module to thehermetically sealed closed-loop flow path when no more liquid extractedfrom the non-reactive gas accumulates in the moisture accumulator. 3.The system according to claim 1, wherein the programmable controller isoperable to: hold the cavity at sub-atmospheric pressure for apredetermined period of time while continuing to apply vacuum pressureto the cavity to remove residual moisture.
 4. The system according toclaim 3, wherein the predetermined period of time is selected to preventoverheating cladding on the radioactive elements.
 5. The systemaccording to claim 3, wherein the programmable controller is furtheroperable to discontinue application of the vacuum pressure to the cavityafter the predetermined period of time has lapsed.
 6. The systemaccording to claim 3, wherein the predetermined period of time is lessthan 2 hours.
 7. The system according to claim 6, wherein thepredetermined period of time is about 30 minutes.
 8. The systemaccording to claim 1, wherein the vacuum module is operable to apply avacuum pressure to the cavity so as to create a sub-atmospheric pressurewithin the cavity to vaporize residual moisture on the spent nuclearfuel elements from the first drying phase.
 9. The system according toclaim 1, further comprising: a dew point temperature sensor operable tomeasure a dew point temperature of the non-reactive gas circulating inthe hermetically sealed closed-loop flow path; the programmablecontroller operably coupled to the dew point measuring sensor andmonitoring moisture in the system during the first drying phase via thedew point temperature sensor; wherein the programmable controller: (1)fluidly isolates the condensing module and gas circulator module fromthe hermetically sealed closed-loop flow path at an end of the firstdrying phase characterized by the dew point measuring sensor obtaining asteady dew point temperature measurement; and (2) fluidly connects thevacuum module to the hermetically sealed closed-loop path to start thesecond drying phase.
 10. The system according to claim 9, wherein thedew point temperature sensor is configured to create signals indicativeof the measured dew point temperature of the non-reactive gas andtransmit the signals to the programmable controller, and wherein theprogrammable controller is configured to analyze the signals and fluidlyconnect the vacuum module to the hermetically sealed closed-loop flowpath via operating the isolation valve when the dew point temperaturemeasurement sensor produces a steady state reading.
 11. The systemaccording to claim 9, wherein the dew point temperature sensor islocated upstream of the condensing module and downstream of the cavityin the hermetically sealed closed-loop flow path.
 12. The systemaccording to claim 1, further comprising: a pressurized non-reactive gasreservoir fluidly coupled to the hermetically sealed closed-loop flowpath upstream of the gas circulator module; and a shutoff valve locatedbetween the non-reactive gas reservoir and the hermetically sealedclosed-loop flow path for controlling addition of the non-reactive gasto the system.
 13. The system according to claim 1, further comprising apreheat module arranged in the hermetically sealed closed-loop flow pathbetween the non-reactive gas reservoir and the gas circulator module.14. The system according to claim 1, wherein the vacuum module isfluidly coupled to the hermetically sealed closed-loop flow path betweenthe gas circulator module and the canister.
 15. The system according toclaim 14, wherein the programmable controller is operably coupled to: afirst shutoff valve located upstream of the canister between the fluidcoupling of the vacuum module to the hermetically sealed closed-loopflow path and the gas circulator module; and a second shutoff valvelocated downward of the canister between the cast and the condensingmodule.
 16. The system according to claim 15, wherein the programmablecontroller moves the first and second shutoff valves from an openposition to a closed position to fluidly isolate the condensing moduleand gas circulator module from the hermetically sealed closed-loop flowpath.
 17. A system for drying spent nuclear fuel in two phasescomprising: a canister defining a cavity configured for holding spentnuclear fuel elements; a gas circulation system comprising in fluidcommunication a condenser module and a gas circulator module; the gascirculation system configured to form a hermetically sealed closed-loopflow path when fluidly connected to the cavity of the canister, the gascirculator module configured to circulate a non-reactive gas in theclosed-loop flow path; an isolation valve operable for selectivelyfluidly connecting a vacuum module to or isolating the vacuum modulefrom the hermetically sealed closed-loop flow path; a programmablecontroller operably coupled to the isolation valve, the gas circulatormodule, and the condensing module; the programmable controllerconfigured to automatically: (1) isolate the desiccant module fromhermetically sealed closed-loop path during a first drying phase; and(2) add the desiccant module as part of the hermetically sealedclosed-loop path during a second drying phase; and a moistureaccumulator which receives moisture from the condenser module, andwherein the programmable controller adds the desiccant module as part ofthe hermetically sealed closed-loop path during the second drying phasewhen no more liquid accumulates in the moisture accumulator.
 18. Thesystem according to claim 17, wherein the programmable controllermonitors moisture accumulating in the accumulator and adds the vacuummodule to the hermetically sealed closed-loop flow path upon theprogrammable controller determining that no more moisture isaccumulating in the accumulator.
 19. The system according to claim 17,further comprising a dew point temperature sensor located in theclosed-loop flow path upstream of the condensing module and downstreamof the cavity, the dew point temperature sensor operable to measure adew point temperature of the non-reactive gas circulating in thehermetically sealed closed-loop flow path.
 20. The system according toclaim 19, wherein: the dew point temperature sensor is configured tocreate signals indicative of the measured dew point temperature of thenon-reactive gas and transmit the signals to the programmable controllerwhich monitors the dew point temperature.